"...convinced her that everyone was out to get her – everyone she loved, everyone she cared about. There were enemies lurking in the shadows, he told her, who would hurt her and make her suffer. She surrendered and pledged allegiance to Balor."
She is called the daughter of Flood and Famine, for she can bring those disasters upon the world.
"She, like her imagined enemies, took to lurking in the shadows breathing fear down the necks of her enemies."
"...has become but a fairy tale story, told to children that she lurks in the shadows, hides behind the doors and waits to spring for those who disobey. She resides in the nightmare world of sleep, coming out to strike fear. The truth is much more terrifying. She is the Bone-Splitter"
It is possible that Bheil was a Callieach that had done horrendous deeds and died. This would make sense as to the power she held and the reference to her being an "ancient evil." She probably aided Soroganth the Flayer in his attempt to crush the light.